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Overview Automate the design of 2D graphical presentations for relational data Why is this hard? –Must express design criteria so that the machine can understand them (expressiveness criteria). –Must express information in an understandable way given the capabilities of the output medium (effectiveness criteria).

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The Graphical Presentation Problem What is an effective encoding of information? Example: Given a car database: Present the Price and Mileage relations. The details about the set of Cars can be omitted. Exponential number of ways of encoding any information.

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Approach: Expressiveness Criteria Graphical presentations are “sentences of graphical languages”, essentially logical statements A set of facts is expressible in a language if and only if it contains a sentence that describes exactly that set of facts: no more, no less.

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Expressiveness Criteria Example: You can describe a 1-D horizontal graph using a “horizontal position” language A graphical sentence can belong to this language if it describes either the horizontal axis or a “+” mark on that axis:

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Expressiveness Criteria Encodes relation: given a language for presenting information (like HorzPos), Encodes is the relationship between the facts you are encoding and the objects on the screen

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Expressiveness Encoding Example: given a relation r with tuples (a i, b i ) (where a is an element of the set of marks and b is an element of the set of positions) for the HorzPos language, there are three Encodes relations: –the range of locations are encoded by the horizontal axis: Encodes(h, {b 1 …b n }, HorzPos) –Each located object o i in the set of marks m encodes an a i Encode(o i, a i, HorzPos)

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Expressiveness Encoding –The relation r can be encoded by the position of each mark along the horizontal axis in the horzPos language Encodes(position(m,h), r, HorzPos)

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Effectiveness Criteria How do we design an effective presentation automatically? Based on empirically verified knowledge, not mathematical rigor. A ranking of perceptual tasks is used to decide which graphical language to employ (Fig 14-15).

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Effectiveness Criteria Principle of Importance Ordering: –Encode more important information more accurately (use information higher in the ranking to encode more important information)

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Composition How do you design new presentation designs? –Can just have a laundry list of them –But it is better to take a bunch of simple “primitive” ones and combine them. Principle of Composition: –Compose two designs by merging parts that encode the same information.

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Axis Composition Example: ozone measurements in two different cities. –Y-axis: ozone density –X-axis: date –first figure, from Yonkers, second from Stamford: overlay them. Only can do this if the axes encode the same information

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Mark Composition Merges mark sets if the sets encode the same information in the same way –position: positions of located objects along the existing axes must be the same –retinal: retinal properties must be the same

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Partition Partition (divide) –order the attributes by importance –divide them up into groups that match expressiveness criteria – can be partitioned into,. – must be repartitioned recursively until something that can be encoded is obtained

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Selection and Composition Selection –For each partition, filter out incompatible design criteria e.g., cannot use maps to encode Composition –Composes the individual designs into a unified presentation of all information